The present invention relates to the testing and inspection of welding seams by means of ultrasonics, and more particularly the invention relates ultrasonic testing of the seam of submerged arc-welded pipes of large and thick pipes (e.g., wall thickness in excess of an inch).
U.S. Pat. No. 4,131,026 granted to one of us with other inventors refers to test procedures of the type referred to above. The method and system as described therein passes the pipe with seam longitudinally through a plurality of test stations being spaced apart (longitudinally as well as laterally) in a well-defined manner, so that different portions of the seam are tested in each instant, but all portions pass adjacent to, or even under, all of the stations. All tests are cyclically repeated in steps, and the individual test results are correlated with each other on the basis of the pipe's advance; these test results are essentially particular transit times of echos (or absence thereof within specifiable periods), amplitude limits, etc. This way, one obtains quite accurately a localized representation of transverse and/or longitudinal defects and flaws as well as of edge zone defects.
The patent referred to above discloses particularly ultrasonic transducers to be disposed directly above the welding seam (or below) and directing test beams, having (a) a component towards the seam, (b) a component in the longitudinal direction of seam and pipe advance, and (c) no component in transverse direction.
Previously and elsewhere, one tests a seam for locating transversely extending flaws by means of test heads, which are also laterally displaced from the seam, but the test beams provide also the components (a) and (b). (See, e.g., U.S. Pat. No. 3,868,847.) Depending on the type of arrangement, one calls those methods V or X test methods. The X and V methods are characterized by the fact that the transmitting transducer cannot serve also as a receiving transducer as far as the detection of a transversely extending crack is concerned, because the cracks (except in very rare instances) will not reflect any significant amount of ultrasonic energy towards the transmitter. For a complete inspection, one needs therefore a comparatively large number of such transducers, particularly when the seam and pipe wall is rather thick.
It should be noted that modern ultrasonic test equipment uses a coupler fluid (e.g., water) between the test heads and the test object. Thus, the test heads do not directly engage the object. It is primarily for this reason that multiple heads are needed in X or V arrangement to make sure that the zones and regions to be inspected are adequately covered.
Whenever one needs, for any reason, separate receivers and transmitters, installation and, particularly, adaptation to different sizes of the test object is made complicated in that the beam path is dependant in each instance upon two heads and their position. Generally speaking, this makes it impossible to provide for a controlled change of but one test parameter while maintaining the others constant. For example, the transmitting transducer head has a particular transmitter lobe along its axis and the receiving transducer has also a particular receiver lobe along its axis. Changing the position and orientation of one requires always a corresponding change of the other because the test requires a particular mutual orientation of these heads to each other, through the test object and the coupler fluid (twice). The first-mentioned patent avoids these problems to some extent, but it was found that thick pipes and correspondingly thick seams are not adequately tested by just two transducers. This is not immediately apparent because seemingly any defect intercepting a test beam propagating into the interior of a seam should produce, e.g., a noticeable and detectable response. Nevertheless, it has been discovered that, for example, flaw detection through pulse echos is more or less limited to surface-near cracks; the X or V methods are not sufficiently sensitive anyway.